1. Field of the Invention
The present invention relates to a semiconductor device having circuits structured with thin film transistors (hereinafter referred to as TFT). For example, the present invention relates to electro-optical devices, typically liquid crystal display panels, and to the structure of electronic equipments loaded with such electro-optical devices as parts. Note that throughout this specification semiconductor device generally indicates devices that acquire their function through the use of semiconductor characteristics, and electro-optical devices, semiconductor circuits, as well as electronic equipments are semiconductor devices.
2. Description of the Related Art
Active matrix type liquid crystal display devices composed of TFT circuits that use polysilicon films have been in the spotlight in recent years. They are the backbone for realizing high definition image displays, in which a plurality of pixels are arranged in a matrix state, and the electric fields that occur in the liquid crystals are controlled in that matrix state.
With this active matrix type liquid crystal display device, as the resolution becomes high definition such as XGA and SXGA, the number of pixels exceeds one million. The driver circuit that drives all of the pixels is extremely complex, and furthermore is formed from a large number of TFTs.
The required specifications for actual liquid crystal display device (also called liquid crystal panels) are strict, and in order for all of the pixels to operate normally, high reliability must be secured for both the pixels and the driver circuit. If an abnormality occurs in the driver circuit, especially, this invites a fault called a line defect in which one column (or one row) of pixels turns completely off.
However, TFTs which use polysilicon films are still not equal to the MOSFETs (transistors formed on top of a single crystal semiconductor substrate), used in LSIs etc., from a reliability point of view. As long as this shortcoming is not overcome, such a view that it is difficult to use TFTs when forming an LSI circuit gets stronger.
The applicant of the present application considers that a MOSFET has three advantages from a reliability standpoint, and infers the reason thereof as follows. A schematic diagram of a MOSFET is shown in FIG. 2A. The MOSFET contains a drain region 201 formed on a single crystal silicon substrate, and an LDD (lightly doped drain) region 202. In addition, there is a field insulating film 203, and a gate insulating film 205 directly under a gate wiring 204.
In that arrangement, the applicant considered that there are three advantages from a reliability standpoint. The first advantage is an impurity concentration gradient seen when looking at the drain region 201 from the LDD region 202. As shown in FIG. 2B, the impurity concentration gradually becomes higher from the LDD region 202 toward the drain region 201 for a conventional MOSFET. This gradient is considered effective in improving reliability.
Next, the second advantage is that the LDD region 202 and the gate wiring 204 overlap. Known examples of this structure include GOLD (gate-drain overlapped LDD), LATID (large-tilt-angle implanted drain), etc. It becomes possible to reduce the impurity concentration in the LDD region 202, the relaxation effect of the electric field becomes larger, and the hot carrier tolerance increases.
Next, the third advantage is that a certain level of distance exists in between the LDD region 202 and the gate wiring 204. This is due to the field insulating film 203 being formed by a shape in which it is slipped under the gate wiring. Namely, a state in which only the overlapped portion of the thick film gate insulating film becomes thick, so an effective relaxation of the electric field can be expected.
A conventional MOSFET compared with a TFT in this way has several advantages, and as a result, is considered to possess a high reliability.
In addition, attempts have been made in which these MOSFET advantages are applied to a TFT. For example, Hatano et al (M. Hatano, H. Akimoto, and T. Sakai, IEDM97 Technical Digest, p. 523-526) realized a GOLD structure that uses sidewalls formed by silicon.
However, compared with a normal LDD structure, the structure published in the paper has a problem in that the off current (the current that flows when the TFT is in the off state) gets large, and therefore a countermeasure is necessary.
As described above, the applicant of the present invention considers that, when the TFT and the MOSFET are compared, the problems associated with a TFT structure affect its reliability (especially its hot carrier tolerance).
The present invention is technology for overcoming this type of problem, and therefore has an object of the invention to realize a TFT that boasts the same or higher reliability than a MOSFET. In addition, another object of the invention is to realize a semiconductor device with high reliability which includes semiconductor circuits formed by circuits using this type of TFTs.
An active layer of the NTFT of the present invention is firstly characterized by including three impurity regions, other than a channel forming region, which have at least three different impurity concentrations. With this, an LDD structure can be obtained, in which the impurity concentration becomes gradually higher away from the channel forming region (in proportion to the distance from the channel forming region). Namely, it is possible to increase the TFT""s reliability by a relieved electric field at the drain edge (vicinity of the border between the drain and the channel forming region).
An aim of the inventor of the present invention is to intentionally form a plurality of regions with the concentration gradient of the LDD section of an exemplary, conventional MOSFET. Therefore, there is no problem with the existence of three or more impurity regions.
Further, a second characteristic of the present invention resides in that it is formed into a state in which the gate wiring (including the gate electrodes) covers (overlaps) at least a part of the LDD region, through the gate insulating film. Deterioration due to a hot carrier can also be effectively suppressed with this type of structure
In addition, a third characteristic of the present invention is that, through the multiplier effect of the first characteristic and the second characteristic described above, the reliability of a TFT can be raised.